Create trigger for postgres database working with QGIS

Hi, I need support with some database issues with QGIS. I need a trigger for update timestamp for spatial features. In my database I've varchar field with datastamp. Default value of the column is NOW(). It woks perfect with new record. If I split spatial feature in QGIS - value of my column in new features values doesn't change. Now, I expect from you to make new trigger which will change timestamp in both features after split.